Image intensifiers for night vision

ABSTRACT

An image intensifier suitable for night vision which employs a channel intensifier, i.e., a plate-shaped resistive matrix provided with conductive coatings on opposite major faces which constitute input and output electrodes respectively connected by elongated passageways in the matrix which constitute secondary emissive channels. A photocathode responsive to infrared and near infrared radiation is positioned opposite the input electrode while a luminescent screen is positioned opposite the output electrode and luminesces under electron impact in the visible spectrum adjoining the infrared portion of the spectrum, i.e., in the red wavelengths.

United States Patent Inventor Pieter Schagen Lisseren, Surrey, England Appl. No. 751,544 Filed Aug. 9, 1968 Patented Jan. 5,1971 Assignee U.S. Philips Corporation New York, N.Y. a corporation of Delaware. by mesne assignments Priority Aug. 10, 1967 Great Britain No. 36717/67 IMAGE INTENSIFIERS FOR NIGHT VISION 2 Claims, 3 Drawing Figs.

US. Cl. 313/94, 250/833, 250/213; 313/101 Int. Cl I-I0lj 39/00 Field of Search 313/103,

92Ph, 108, 109, 94, 95, I01; 250/213VT, 83.3IRI

[56] References Cited UNITED STATES PATENTS 2,905,844 9/1959 Sternglass Primary Examinerlames W. Lawrence Assistant Examiner-David OReilly Atmmey-Frank R. Trifari PAT'ENTEU'JAN 519m 0 3553518 sum 1 or z IN VENTOR. PIE TER SCHAGHV PATENTEDJAN 5:91: 3553518 sum as? 2 flg. 2

INVENTOR. PIETH? SCHAGEN IMAGE INTENSIFIERS FOR NIGHT VISION This invention relates to electron multiplier and image intensifier devices. More particularly the invention relates to channel intensifier" devices and to electronic tubes employing such devices. 1

A channel intensifier device is a secondary emissive electron multiplier device for an electronic tube which device comprises a resistive matrix in the form of a plate the major surfacesof which constitute the input and output faces of the matrix, a conductive layer on the input face of the matrix serving as an input electrode, a separate conductive layer on the I I electrodes to the other face of said assembly. Secondary emission multiplication takes place in the channels.

With' such devices the distribution and cross sections of the channels" and the resistivity o f the matrix are such that the' resolution and electron multiplication characteristic of any one unit area of the device is sufficiently similar to that of any other unit area for any imaging purposes envisaged.

If such a device is used in an imaging tube or system, the

latter will be referred to for convenience as an image intensifier" tube or system rather than as an image converter tube or system even in applications where the primary purpose is a change in the wavelength of the radiation of the image.

In such image intensifier tubes 21 luminescent display or output screen is used containing a phosphor with the highest possible luminous efficiency in order to produce the maximum brightness gain. For this purpose the phosphors generally used for the screen have a luminescence peaking in the green-toblue part of the spectrum, in order to coincide more or less with the maximum in the spectral sensitivity curve of the eye.

With the advent of channel intensifier devices it has been found that such image intensifier-tubes are suitable for observation at night. Due to the high luminous efficiency' of the phosphor and the high electron gains possible with such devices 1 the luminescence of the screen may exceed considerably the low light level to which the observers eye is adapted when he is not looking at the intensified image on the phosphor screen. 1

It is an object of the invention, when image intensification is used for observation at night, to avoid disturbance of the dark adaptation of the observer looking at the intensified image on the phosphor screen. i i

The present invention provides an electronic image intensifier tube suitable for night vision which tube comprises a channel intensifier device as herein defined a photocathode on the input side of said channel intensifier device and a luminescent viewing screen on the output side of saidchannel intensifier device and according to the invention the screen is adapted to luminesce entirely or mainly in the red parts of the visible spectrum.

By using such a tube for night vision itis possible for an observer to remain dark-adapted since'red light affects the dark adaptation far less than green or blue light.-

The'tube may be one wherein the viewing screen employs a phosphor of the zinc-phosphate manganese-activated type known generally as the P27 type.

The photocathode is chosen forsensitivity to radiation of the wavelengths principally present at night. This chosen sensitivity characteristic of the photocathode may differ for different applications and, in particular, the characteristic may include infrared wavelengths in cases in which it is desired to 2 In a preferred arrangement the photocathode is of the multialkali type generally known asS20.

The improvement obtainable in a practical case will now be described by way of example with reference to FIGS. 1 and 2 of the accompanying drawings which illustrate respectively the spectral sensitivity cures of the human'eye and those of a P27 hosphor screen.

a In [(3. l are shown spectral sensitivity curves of the eye for photopic and scotopic vision indicating relative response as a function of the wavelength insofar visible radiation is concerned. FIG. 2 shows the spectral emission characteristic of a P27 phosphor indicating relative radiant energy as a function of the wavelength. f v

FIG. 3 shows an image intensifier for night vision according to the invention.

One lumen of P27 light, as defined by the photopic sensitivity curve 1 of the cones, corresponding to the nondark-adapted eye, has the effect of only one twenty-fifth lumen on the rods. which are normally operative in the dark-adapted eye with the scotopic sensitivity curve 2. This expresses numerically the fact that the rods are much less sensitive to red lightthan the cones as shown by the curves. It means that the dark-adapted eye can be presented with redlightfrom a P27 phosphor at a brightness level, expressedin foot-Lamberts, iwhich is about 25 times higher than when it is presented with white light, before the dark adaptation is impaired to the same extent.

A picture on a P27 phosphor screen can thus be viewed with the aid of the cones, and seen as a .red" picture of acceptable brightness, without unduly overloading the rods. When the observer subsequently looks away from the screen at a scene with a much lower brightness level, but illuminated with white light, he can immediately use his scotopic vision. An image intensifier of the usual type with theimprovementstherein is shown in FIG. 3 and includes an envelope 1, a photocathode 2 principally responsive to wavelengths in the near infrared and infrared regions of the spectrum, a channel intensifier 3 comprising a resistive matrix having conductive coatings on opposite faces constituting i nput and output electrodes 4 and 5 respectively connected by elongated channels 6 the walls of observe radiant heat from any warm or heated objects that may be present.

which are secondary emissive thus providing secondary emissive channels, and a luminescent screen 7 which luminesces under electron impact in the red region of the visible portion of the spectrum. i

As a modification of the invention, the red luminescent viewing screen can be replaced by the combination of a viewing screen which, insofar as visible radiation is concerned, is not adapted to luminesce entirely or mainly in the red parts of the visible spectrum, and a red filter through which said screen can be viewed. Such a screen may, for example, be adapted toluminesce in white or substantially in white. j

I claim: I Y 1. An electronic image intensifier tube suitable for night vision which tube comprises a plate-shaped resistive matrix the major surfaces of which conslituteinput and output facesof the matrix, a conductive laye on the input face of the matrix serving as an input electrode, a separate conductive layer on the output surface serving as an output electrode, said matrix having a plurality of elongated channels each providing a passageway connecting said input and output electrodes with secondary emission taking place in said channels, an infrared sensitive photo-cathode on the input side of said matrix, and a luminescent viewing screen oh the output side of said matrix,

said screen luminescinguponlelectron excitation substantially only in the red region of the visible portion of the spectrum.

2. A tube as claimed in claim '1 wherein the viewing screen employs a phosphor of the zinc phosphate manganese-ac tivated type. 

1. An electronic image intensifier tube suitable for night vision which tube comprises a plate-shaped rEsistive matrix the major surfaces of which constitute input and output faces of the matrix, a conductive layer on the input face of the matrix serving as an input electrode, a separate conductive layer on the output surface serving as an output electrode, said matrix having a plurality of elongated channels each providing a passageway connecting said input and output electrodes with secondary emission taking place in said channels, an infrared sensitive photo-cathode on the input side of said matrix, and a luminescent viewing screen on the output side of said matrix, said screen luminescing upon electron excitation substantially only in the red region of the visible portion of the spectrum.
 2. A tube as claimed in claim 1 wherein the viewing screen employs a phosphor of the zinc-phosphate manganese-activated type. 